Study On Stepwise Utilization Of Valuable Elements In Circulating Fluidized Bed-derived Fly Ash

Coal gangue,discharged in the process of coal mining,is one of the largest industrial solid wastes in China.In 2014,China promulgated the“Management Measures for Comprehensive Utilization of Coal Gangue”,advocating the use of coal gangue for combustion and power generation in Inner Mongolia,Shanxi,Shaanxi and Henan.utilization of coal gangue for burning power generation not only to achieve the bulk use of coal gangue,but also to alleviate the environmental pressure brought by the coal gangue stockpile.CFB fly ash emissions are growing every year as solid waste from the burning process of coal gangue,but utilization efficiency is still low.The unused CFB fly ash accumulates in large quantities,which seriously threatens the ecological environment.The coal gangue in north Shanxi and eastern Inner Mongolia is rich in valuable elements such as Al,Li and Ga.These valuable elements are finally enriched in CFB fly ash.Therefore,valuable metals such as Al,Li and Ga in the CFB fly ash are considered as an important resource.To improve the utilization of valuable elements in CFB fly ash,most studies have focused on the leaching of valuable elements such as aluminum,lithium,and gallium from CFB fly ash by acid leaching.However,the influence of leaching conditions on the leaching behavior of valuable elements is not clear,and there is still a problem of producing secondary solid residue after acid leaching.It is interesting found that the residue can be used as raw materials for porous adsorption material due to the formation of many mesoporous during the process of extracting valuable elements such as Al,Li and Ga from CFB fly ash.In this work,it is not only realizes the high-value utilization of CFB fly ash but also achieve“reuse,reduce and harmless”that both of the extraction of valuable elements and preparation of mesoporous materials.based on above mentioned,this paper investigates the leaching behavior of Al,Li and Ga in hydrochloric acid solution from CFB fly ash and used its residue as raw material to produce adsorbent.The adsorption performance of methylene blue and Pb²⁺by as-obtained adsorbent was investigated.The main conclusions are as follows:（1）The valuable elements in CFB fly ash are leached through a stepwise method including first acid（HCl）and then alkali（NaOH）and finally acid（HCl）after mechanical activation.The results show that the leaching efficiency of Al,Li and Ca in CFB fly ash is up to 83%,78%and 69%,respectively under the optimal leaching conditions.The leaching behaviors of all of the above-mentioned metals is belong to the shrinking core model,to be specific,the leaching rate before and after 20 min is controlled by the chemical reaction of surface and H⁺diffusion,respectively;during the process of alkali/second acid,the leaching behavior of silicon and aluminum is controlled by the diffusion of OH^-and H⁺,respectively.（2）With the dissolution of aluminum,lithium,and gallium during the process of injected into acid,the residue with slit-shaped mesopores structure was formed.The residue after second acid leaching can be used as an efficient adsorbent for removing methylene blue from dye wastewater due to its specific surface area reaches 273 m²/g,which has a significant increase compared with CFB fly ash（12 m²/g）.The maximum adsorption capacity is up to 140 mg/g under the optimal leaching conditions.The adsorption of methylene blue was in accordance with the Langmuir model and pseudo-second order kinetic model,which indicated that the process was dominated by monolayer chemical adsorption.（3）In this work,a zeolitic material with Na-P1 zeolite as the main phase was successfully synthesized via a hydrothermal method by using HR-1 as the raw material,and was employed as an adsorbent to remove Pb²⁺from aqueous solution.The adsorption capacity of Pb²⁺by as-obtained zeolite was influenced by the amount of Na-P1 and that micromorphology.The optimum synthesis conditions of zeolite using residue after the first acid as raw materials are as follows:mass is 10 g,temperature is110℃,time is 9 h,CTAB dosage is 1 g,the volume NaOH is 100 m L（2 mol/L）.The adsorption capacity of Pb²⁺by as-obtained zeolite was up to 424.69 mg/g under the optimal conditions:p H=6and time are 60 min etc.The removal efficiency of Pb²⁺can reach 100%when the initial concentration of Pb²⁺is 100–300 mg/L.The adsorption process and kinetics of Pb²⁺by the obtained zeolitic material can be described by Langmuir isotherm and pseudo-second-order kinetic models,which indicated that the adsorption process is mainly monolayer chemisorption,the adsorption equilibrium time decreases with the increase of temperature,suggesting that the increase of temperature is helpful to the adsorption,and the adsorption process is endothermic.XPS analysis revealed that the ion exchange between Pb²⁺and Na⁺is the main adsorption mechanism.Therefore,utilization of CFB fly ash acid leaching residue to synthesize zeolite adsorbent material for adsorption of Pb²⁺in solution is a cost-effective way.